1. Field of the Invention
The present invention relates to an apparatus for supplying breathable gas to a human, used in, for example, Continuous Positive Airway Pressure (CPAP) treatment of Obstructive Sleep Apnea (OSA), other respiratory diseases and disorders such as emphysema, or the application of assisted ventilation.
2. Description of Related Art
CPAP treatment of OSA, a form of Noninvasive Positive Pressure Ventilation (NIPPV), involves the delivery of a pressurized breathable gas, usually air, to a patient's airways using a conduit and mask. Gas pressures employed for CPAP can range, e.g., from 4 cm H2O to 28 cm H2O, at flow rates of up to 180 L/min (measured at the mask), depending on patient requirements. The pressurized gas acts as a pneumatic splint for the patient's airway, preventing airway collapse, especially during the inspiratory phase of respiration.
Typically, the pressure at which a patient is ventilated during CPAP is varied according to the phase of the patient's breathing cycle. For example, the ventilation apparatus may be pre-set, e.g., using control algorithms, to deliver two pressures, an inspiratory positive airway pressure (IPAP) during the inspiration phase of the respiratory cycle, and an expiratory positive airway pressure (EPAP) during the expiration phase of the respiratory cycle. An ideal system for CPAP is able to switch between IPAP and EPAP pressures quickly, efficiently, and quietly, while providing maximum pressure support to the patient during the early part of the inspiratory phase.
In a traditional CPAP system, the air supply to the patient is pressurized by a blower having a single impeller. The impeller is enclosed in a volute, or housing, in which the entering gas is trapped while pressurized by the spinning impeller. The pressurized gas gradually leaves the volute and travels to the patient's mask, e.g., via an air delivery path typically including an air delivery tube.
There are currently two common ways in which the blower and impeller can be configured to produce the two different pressures, IPAP and EPAP, that are required in an ideal CPAP system. A first method is to set the motor/impeller to produce a constant high pressure and then employ a diverter valve arrangement that modulates the high pressure to achieve the required IPAP and EPAP pressures. CPAP systems according to the first method are called single-speed bi-level systems with diverters. A second method is to accelerate the motor that drives the impeller to directly produce IPAP and EPAP pressures. CPAP systems according to the second method are called variable-speed bi-level systems.
Variable-speed bi-level CPAP systems have a number of particular disadvantages. A first disadvantage is that in order to switch rapidly between IPAP and EPAP, the impeller must be accelerated and decelerated rapidly. This causes excessive stress on the impeller, motor, and bearings. However, if the impeller is accelerated slowly, the pressure rise may be unsatisfactorily slow, and thus, the patient may not receive adequate treatment.
Rapid acceleration and deceleration of the motor and impeller also result in excessive heat generation and undesirable acoustic noise. (“Undesirable” acoustic noise, as the term is used here, refers to acoustic noise that is overly loud, as well as acoustic noise which occurs at a frequency that is irritating to the user, regardless of its volume.) In addition, design engineers are often forced to make a compromise, sacrificing optimal pressure and flow characteristics in favor of achieving a desired peak pressure.